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Crystallized Melt Inclusions in Quartz: tiny windows into magmatic evolution

Fazilat Yousefi1, David R. Lentz1

Abstract

Key words: Crystallized melt inclusion, cMIs, ChromaSEM-CL imaging, μXRF-EDS Crystallized melt inclusions (cMIs), or nanogranitoids, are tiny silicate melt pockets trapped in minerals like quartz during crystallization. They provide crucial insights into magmatic systems, across volcanic, subvolcanic, and plutonic environments, excluding volatile contents. As the rock cools, most inclusions crystallize, except very small ones (typically <100 µm for older systems), preserving mineral assemblages that reflect composition and conditions of original melt. Crystal-rich clots gained popularity, but is broader than cMIs. cMIs, along with phenocrysts, microphenocrysts, and their clusters, are useful tools for understanding magmatic evolution. Various analytical methods, including petrographic and ChromaSEM-CL imaging of phenocrysts like quartz (elemental mapping of phases) reveals fine-scale features and zoning patterns in quartz that provides context for trapping cMIs, providing insights into their composition and post-entrapment crystallization (PEC) processes. This technique helps identify post-entrapment alteration -- modification of cMIs, providing insight into the preservation of the original magmatic signature. Optical imaging further complements this by offering detailed visual assessments of cMI integrity. SEM-EDS provides high-resolution imaging and elemental analysis of cMIs, revealing their detailed chemical composition. μXRF-EDS with LA-ICPMS allows for spatially resolved chemical mapping of cMIs and surrounding minerals, helping to identify compositional gradients and melt heterogeneities. Petrographic and compositional analysis of cMIs in quartz and phenocrysts reveal critical insights into magmatic processes like boundary layers, magma mixing, and fractional crystallization to cooling rate affecting relative crystallinity. Their compositional variations reveal magma evolution, tracing origin and changes from mantle to surface during ascent.